1. Field of the Invention
This invention relates to use of broadband communications for a network. More specifically, the invention relates to use of spread spectrum communications on a noisy network media such as a powerline using carrier sensing protocols.
2. Description of the Prior Art
The use of spread spectrum communications for more reliable and secure communications is well-known. By transmitting an information signal over a frequency spectrum that is broad with respect to the information bandwidth, and in a manner that can be decoded/despread at the receiver, several benefits are realized. First, any particular narrow band frequency impairments due to interference or attenuation will not necessarily impair the received signal because of the redundancy of spectrum used for the information signal. Secondly, the encoding/spreading technique can be chosen such that performing the inverse function (decoding/despreading) on the received signal effectively spreads any received interfering signal or "noise" in the process, thereby minimizing the impact of such noise at the receiver.
Spread spectrum for data communications, in its well known form, can be achieved in several ways. Typically, the methods used are classified as variations of the "direct sequence" or "frequency hop" techniques. (See, for instance, Spread Spectrum Systems, Second Edition, by Robert C. Dixon, John Wiley & Sons, 1984.) Each of these methods share the requirement of a synchronization process. This process must take place in order to establish a connection between the transmitter and receiver. The purpose of this synchronization is to allow the transmitter and receiver to follow the same encoding and decoding process in time synchronization--whether it be code transitions of direct sequence type modulation or frequency hop transitions. This synchronization process can be accomplished in a number of well-known ways. Once synchronization is established between a transmitter and a receiver, data is generally transferred by modulating the higher frequency encoding/spreading signal with the information signal.
As long as the transmitter and receiver stay in synchronization, the data communications capability of the link is enhanced by the spreading function of the encoding signal. Since the receiver is locked in time to the encoding pattern, the receiver averages out over a period of time (correlates) the encoded signal through possible interference, and then demodulates the received information signal from the recovered encoding signal. Interfering signals are first spread by the decoding process, and then filtered by the correlation/demodulation process. The ability to reject high levels of interfering signals is one of the primary benefits of spread spectrum communications.
It is the encoding characteristic that also allows spread spectrum communications to be more secure. By selecting an encoding and synchronization process based on a psuedo-random number sequence of long length, which sequence is known only to the intended receiver, a transmitter can establish communications with the intended receiver that is difficult for any eavesdropping receiver to synchronize to, since the code sequence is unknown to the eavesdropper. In network applications, however, such secure communications are not usually required, at least at the physical level of transmission.
The synchronized communication characteristic of spread spectrum is also why spread spectrum is believed to be unsuitable for carrier-sense based networks--those that allow multiple access to a media using carrier-sense techniques for contention resolution and collision detection (e.g., CSMA/CD networks such as Ethernet, and the proposed EIA CEBus network). The reason that the prior art spread spectrum is unusable in these connectionless packet-oriented carrier-sense based networks, is that spread spectrum is basically a "connected" communications protocol that to the extent possible ignores the rest of what is happening when a "connection" has been established. A transmitter, even with the same encoding technique, will not be received by a receiver that is synchronized to and receiving from another transmitter, if it is at least marginally out of phase in time (out of synchronization). In fact, a common "network" technique with spread spectrum communications is to use Code Division Multiple Access. Simply put, this is a number of "connected" (synchronized) transmitters and receivers sending and receiving simultaneously, each "connection" using a different code sequence, unaware of the existence of the others. In fact they can all be using the same encoding sequences as long as any transmitter/receiver pair is synchronized to a different point in the encoding sequence than is any other pair at any point in time.
In a network where the media carrier signal is managed and only one transmitter is normally allowed to operate at one time, techniques of contention-resolution and collision-detection are used to arbitrate the use of the carrier on the media. These techniques depend on the ability of any receiver to detect the presence of a signal "carrier" on the media at any instance in time. In spread spectrum this is not practical, since only if synchronization is achieved can the system determine the presence of a "carrier", and then once synchronization is achieved, no other "carrier" (interference-collision) will be detected. Typically, this synchronization process is non-trivial, and requires time to achieve well beyond that allowed for "carrier-detect" in such networks.
The networks referred to here may reside on any media that allows managed multiple accesses. In the case of typical LAN-type networks, such as Ethernet, the media usually consist of a coaxial cable and connections meeting certain stringent requirements. In this case, the use of spread spectrum communications would typically not be beneficial, since the media is typically well behaved electrically and relatively noise-free. In the case of other media, however, where the environment is not so well behaved or easily controlled, or is potentially noisy or suffers from variable attenuation or other flaws, the benefits of spread spectrum communications could be substantial.
The transmission characteristics of an AC powerline (a typically noisy media) often exhibit narrow band impairments. Most previously known powerline carrier communications systems utilize a single carrier frequency (ASK modulation) or a narrow band of frequencies (FSK modulation), thus making existing systems susceptible to these commonly found narrow band impairments. If these impairments approximately line up with the carrier frequency the system no longer functions properly. The ASK method of modulation, usually at 120 KHz, is by far the most common method in use today especially in the residential (consumer) market.